Recently, downsizing of electric and electronic appliances is advanced, and magnetic cores of small size and high performance are demanded. In a choke coil used at high frequency, a ferrite core and a dust core are used. Of them, the ferrite core is noted for its defect of small saturation magnetic flux density. By contrast, the dust core fabricated by forming metal magnetic powder has an extremely large saturation magnetic flux density as compared with the soft magnetic ferrite, and it is therefore advantageous for downsizing. However, the dust core is not superior to the ferrite in magnetic permeability and electric power loss. Accordingly, when the dust core is used in the choke coil or inductor core, the core loss is large, and hence the core temperature rise is large, so that it is hard to reduce the size of the choke coil.
The core loss consists of eddy current loss and hysteresis loss. The eddy current loss increases in proportion to the square of frequency and the square of a flowing size of eddy current. Therefore, in the dust core used in the coil, to suppress generation of eddy current, the surface of the magnetic powder is covered with an electric insulating resin. However, in order to increase the saturation magnetic flux density, the dust core is formed usually by applying a forming pressure of 5 tons/cm.sup.2 or more. As a result, the distortion applied to the magnetic material is increased, and the magnetic permeability deteriorates, while the hysteresis loss increases. To avoid this, after forming, heat treatment is carried out as required to remove the distortion.
The dust core requires an insulating binder in order to keep electric insulation among magnetic powder particles and to maintain binding among magnetic powder particles. As the binder, an insulating resin or an inorganic binder is used. The insulating resin includes, among others, epoxy resin, phenol resin, vinyl chloride resin, and other organic resins. These organic resins, however, cannot be used where high temperature heat treatment is required for removal of distortion because they are pyrolyzed during heat treatment.
Conventionally, various inorganic binders have been proposed, including silica water glass, alumina cement disclosed in Japanese Laid-open Patent No. 1-215902, polysiloxane resin disclosed in Japanese Laid-open Patent No. 6-299114, silicone resin disclosed in Japanese Laid-open Patent No. 6-342714, and a mixture of silicone resin and organic titanium disclosed in Japanese Laid-open Patent No. 8-45724.
In the conventional ferrite core, in order to suppress the decline of the inductance L value in direct-current superposing and assure the direct-current superposing characteristic, a gap of several hundred microns is provided in a direction vertical to the magnetic path. Such wide gap, however, may be a source of beat sound, or when used in a high frequency band, in particular, the leakage flux generated in the gap may extremely increase the copper loss in the winding. On the other hand, the dust core is low in magnetic permeability and is hence used without gap, and therefore it is small in beat sound and copper loss due to leakage flux.
In the core having a gap, the inductance L value declines suddenly from a certain point in the direct-current superposing current. In the dust core, by contrast, it declines smoothly along with the direct-current superposing current. This is considered because of the presence of the distribution width in the magnetic space existing inside the dust core. That is, at the time of press forming, a distribution width is formed in the distance among magnetic powder particles isolated by a binder such as resin and in the magnetic space length. The magnetic flux begins to short-circuit and saturate from the position of shorter magnetic space length or from the closely contacting position of magnetic powder particles, which is considered to cause such direct-current superposing characteristic. Therefore, in order to assure an excellent direct-current superposing characteristic securely, by increasing the amount of the binder, it is necessary to keep a magnetic space in a size more than the required minimum limit. However, when the content of the binder is increased, the magnetic permeability of the entire core is lowered. Besides, if the core loss is large in the high frequency band, although the apparent direct-current superposing characteristic is excellent, it is only that the apparent magnetic permeability is increased when the core loss is larger. It is hence difficult to satisfy the contradictory properties of small core loss and excellent direct-current superposing characteristic at the same time.